1. Field of the Invention
This invention relates to a process for the removal of organics which are acutely toxic to marine life and malodorous from the waste water of petrochemical operations. More particularly, the invention is directed to the removal of naphthalenic compounds, the constituents of petrochemical plant waste water which have been found to contribute most strongly to odor in the waste water and which have been found to be acutely toxic to marine life.
2. DESCRIPTION OF THE PRIOR ART
As concern for the maintenance of a healthy unpolluted environment has grown, clean water quality criteria have become increasingly stringent. Of particular concern has been the effect of waste water constituents from petroleum refining and petrochemical operations, which constituents may be malodorous and/or acutely toxic to marine life.
Almost all of the hydrocarbons present in the waste water from petroleum refining and petrochemical operations have been found to be toxic to marine life at high concentrations. Similarly, all of the constituents have been found to contribute to the odor of the water. For example, benzene in concentrations of about 10 mg/l was found to be toxic to marine life. Toluene and xylene were found to be toxic at concentrations of about 20 mg/l. In contrast, naphthalenes have been found to be toxic to marine life in concentrations as low as 2 mg/l (which is equivalent to 2 ppm).
Several methods have been considered for more efficient treatment of waste water. Of these methods, removal of the residual organic compounds from waste water by activated carbon adsorption has received much attention.
E. G. Paulson in a paper entitled, "Adsorption as a Treatment of Refining Effluent," investigated the technical and economic feasibility of treating refinery effluent by means of a granular activated carbon waste treatment process. The results of that study were that granular activated carbon treatment can produce a refinery effluent containing less than 20 mg/l of BOD (biochemical oxygen demand) and less than 10 units of RON (recognition odor number) from either primary or secondary refinery effluents. Paulson further concluded that those organic compounds contributing to odor are preferentially adsorbed relative to total mixture of organic contaminants. From this it was concluded that, based on a desired effluent quality of less than 20 mg/l of BOD and less than 10 units of RON, the controlling constituent in the operation of an adsorption system will be the BOD.
D. M. Giusti, R. A. Conway and C. T. Lawson investigated the adsorptive capacities of various carbons for specific organic compounds. Among the compounds investigated were alcohols, aldehydes, amines, pyridines, esters, ethers, ketones, organic acids and a number of aromatic compounds. The aromatics investigated were benzene, toluene, ethylbenzene, phenol, hydroquinone, aniline, styrene, and nitrobenzene. There was no investigation, however, of fused ring aromatic compounds such as naphthalene. The result of this study showed that as molecular weight increases and as polarity, solubility and branching decrease, the degree of adsorption onto activated charcoal increases predictably.
In a paper entitled "Limitations of Activated Carbon Adsorption for Upgrading Petrochemical Effluents," C. T. Lawson and John A. Fischer concluded that excellent removel of MEP and thio-resistant polyethers was still being achieved after a significant breakthrough of the gross parameter COD (chemical oxygen demand) has occurred. Additionally, it was found that "foamability" was suppressed. From this it was concluded that polyethers could still be removed and "foamability" could be suppressed when the net adsorptive capacity of the carbon for COD and MEP had been exhausted.
While prior art processes have shown excellent results in removing hydrocarbon pollutants from the effluent waste water in petroleum refining and petrochemical operations, a serious drawback to the conventional activated carbon adsorption disclosed in those processes has been the long contact times required to achieve satisfactory reduction of hydrocarbons. These long contact times have necessitated regeneration of large quantities of activated carbon and have added measurably to the overall treatment cost.
In accordance with the present invention, it has been found that the regeneration costs for the removal of acutely toxic and malodorous naphthalenic compounds can be greatly reduced by contacting the waste water for short contact time periods with activated carbon which has become substantially saturated with respect to its ability to remove non-naphthalenic compounds. It has been found that naphthalenic compounds continue to be adsorbed after the capacity of the carbon to adsorb non-naphthalenic compounds has become exhausted. It has also been found that these naphthalenic compounds are adsorbed very rapidly so that reduction of the level of naphthalenic compounds to below a predetermined acceptable toxicity level, i.e., less than 2 ppm, is rapidly achieved.
The process of the claimed invention employs short contact times, taking advantage of the tendency of naphthalenic compounds to be rapidly adsorbed. Since acceptable toxicity levels can be achieved with short contact times, longer contact times, which would not provide any meaningful improvement in toxicity removal are avoided. Hence the expense associated with regeneration of the carbon is reduced.